Current collector



Dec. 29, 1,97@ ELLIQTT y 3,551,319

` CURRENT COLLECTOR Filed Sept. 6. 1968 imam/4( ELL/arr I NVENTOR.

` 'BYmMTW United States Patent O U.S. Cl. 204--243 4 Claims ABSTRACT FTHE DISCLOSURE The instant invention relates to an electrolytic cell forthe production of aluminum. The cell comprises a shell having an anodesuspended therein. A carbonaceous lining is provided for the shell. Oneor more cathodic current collectors are embedded in the lining. Aferrous sheath having a copper core is utilized for the cathodic currentcollector so that the major portion of the current during cell operationwill flow out of the cell in the copper core and the ferrous sheathfunctions as a shield to reduce magnetic field of the collector at thepoint of interaction with the molten metal pad which forms a liningduring the operation of the cell. This reduces undesirable movements andunequal elevation of the molten metal pad formed on the lining duringcell operation.

BACKGROUND OIF THE INVENTION The metal aluminum is extracted fromaluminum-bearing compounds such as alumina (A1203) by electrolysis froma molten salt bath or electrolyte. In the production of aluminum by theconventional electrolytic process, commonly referred to as theHall-Heroult process, the electrolytic cell comprises in general asteelshell having disposed therein a carbonaceous lining. The bottom of thecarbonaceous lining together with a layer of electrolytically producedmolten aluminum which collects thereon during operation serves as thecathode. One or more consumable carbon electrodes is disposed from thetop of the cell and is immersed at its lower extremity into la layer ofmolten electrolyte which is disposed in the cell. The carbon electrodesare connected to an anode bus bar which in turn is connected to a sourceof current supply. In the bottom of the carbon lining are one or morecathodic current collectors which are connected to a cathode bus barwhich in turn is also connected to the source of current completing thecircuit. In operation, the electrolyte or bath which is a mixture ofalumina and cryolite is charged to the celll and an electric current ispassed through the cell from the anode `to the cathode via the layer ofmolten electrolyte. The alumina is dissociated by the current so thataluminum is deposited on the liquid aluminum cathode and oxygen isliberated at the carbon anode, forming carbon monoxide and carbondioxide gas. A crust of solidified electrolytev and alumina forms on thesurface ofthe bath, and this is usually covered over with additionalalumina.

In the conventional electrolytic process, use has been made of Atwotypes of electrolytic cells, namely that commonly referred to as apre-bake cell and that commonly referred'to as a Soderberg cell. Witheither cell, the reduction process involves precisely the same chemicalreactions. The principal difference is one of structure. In the pre-bakecell, the carbon anodes are pre-baked before 3,551,319 Patented Dec. 29,1970 being installed in the cell, while in the Soderberg or selfbakinganode cell, the anode baked in situ; that is, it is baked duringoperation of the electrolytic cell, thereby utilizing part of the heatgenerated by the reduction process. The instant invention is applicableto either cell.

The effects of magnetic fields upon the molten metal pad are well knownand have long been a problem to the aluminum industry. A detaileddiscussion of them may be found in such prior art as U.S. Pat. 2,874,110to Thayer. The current flow in the molten aluminum pad departs from avertical path due to the relative resistances between itself and thecarbon lining and the cathodic current collector, which normally hasbeen a solid iron collector bar, thus creating a reaction force betweenthe metal pad and the collector bar causing movements of the metal. Theeffects of the magnetic fields upon the metal pads result in varyingelevations of metal, pinch effect, and possible motoring or rotarymotion of the metal within the pot. These movements of the molten metalpad make it very difficult to control the anode-cathode distance foroptimum operation of the cell.

Exactly why this occurs is not really known. However, as discussed indetail in the Thayer patent referenced above, it is believed that theprincipal contributor to this undesirable molten metal pad movement isthe eld from the cathode collector bars reacting with the fields of themetal pad. It is known that the resulting forces of reaction of twomagnetic fields are a function of the ux intensity and the angle ofincidence of the two fluxes. Because of this, it has been assumed thatthe field from the collector bars being very close to the metal padwould be the dominant inuence.

The prior art contains references to many attempts to solve thisproblem. These various attempts have had varying degrees of success. Thegeneral approach to the problem seems to have been to try variousarrangements ,of collector bars in the cell lining or shapes ofcollector bars to try to alleviate the situation.

SUMMARY oF THE INVENTION The instant invention solves this problem whichhas so long plagued the industry by providing in an electrolytic cellfor the production of aluminum which comprises a shell with acarbonaceous lining therein and anode suspended therein and having acathodic current collector embedded in the lining, the improvement whichcomprises utilizing for the cathodic current collector a ferrous sheathhaving a copper core so that the major portion of the current will flowout of the cell in the copper core and the ferrous sheet functions as ashield to reduce the magnetic eld of the collector at the point ofinteraction with the molten metal pad which forms on the lining duringthe operation of the cell. The undesirable movements and unequalelevation of the molten metal pad formed on the lining during celloperation are reduced by this invention. The ferrous sheath may be inthe form of a hollow cylinder with the copper core in the form of asolid cylinder disposed within the ferrous sheath. However, for ease ofassembly and disassembly, and to prevent magnetic saturation, it isdesirable that the ferrous sheath have a substantially downwardlyopening U shaped cross section and the copper core be disposed withinthe bight of the' U. In this embodiment the cathodic current collectormay be horizontally disposed in the conventional manner.

BRIEF DESCRIPTION OF DRAWING FIG. 1 is a transverse elevation view insection of a reduction cell embodying principles of the instantinvention. FIG. 2 is a sectional view of part of the cathode of the cellof FIG. 1 taken along line 2-2 of FIG. 1 showing one structure of thecathodic current collector according to the instant invention.

FIG. 3 is a view similar to FIG. 2 showing a variation on the structureof the cathodic current collector.

DETAILED DESCRIPTION One embodiment of a reduction cell or potillustrating the instant invention is shown schematically in FIG. 1. Inthis embodiment, is a metal shell generally steel, Within which isdisposed in the usual manner an insulating layer 12 which can be anydesired material, e.g., alumina, bauxite, clay, aluminum silicate brick,etc. Within the insulating layer 12 is disposed cell lining 14 which hasto have carbon bottom when you use collector bars and can be of anydesired material, e.g. carbon, alumina, fused alumina, silicon carbide,silicon nitride, bonded silicon carbide or other desired materials. Atleast the portion of the lining 14 in the bottom of the cell is of acurrent conducting material. Most commonly, the lining is madeup of aplurality of carbon blocks or is rammed carbon mixture or a combinationof the rammed carbon mixture for the bottom of the lining with side andend walls constructed of blocks of carbon. Alternatively, the side andend walls can ybe constructed of blocks of silicon carbide or othersuitable refractory. The lining 14 defines a chamber which contains apool of molten aluminum 16 and a body of molten electrolyte or bath 18,as described.

When the cell is in operation so that aluminum is being produced,electrolyte 18 and aluminum pool 16 are both in the molten state.Suspended from above the electrolyte, and partially immersed therein areone or more anode electrodes 20 of the conventional carbon type whichcan be either of the pre-bake or Soderberg (selfbaking) type known tothe art. Molten electrolyte 18 is covered by a crust 22 which consistsessentially of frozen electrolyte constituents and additional alumina.As alumina is consumed in electrolyte 18, the frozen crust isperiodically broken and more alumina fed into the electrolyte.

The anode is connected by suitable means (not shown) to the positivepole of a source of supply of electrolyzing current. For purposes ofcompleting the electric current,

use is made of cathodic current conducting elements or l collectors 24.As shown herein the cathodic current collector 24 has a copper core 25.The copper cores of the cathodic current collectors extend throughsuitable openings provided in the metal shell, insulation layer andlining with the inner end thereof projecting into the lining. The outerends of each of the elements 24 are connected by suitable means to thenegative or cathode bus bars 26. With reference now also to FIG. 2wherein the structure of the cathodic current collector assembly 24 canbe seen in greater detail it will be seen that copper core 25 of theassembly is disposed with a ferrous sheath 28.

As shown, ferrous sheath 28 has a substantially downwardly opening Ushaped cross section and copper core 25 is disposed within the bight ofthe U. In the embodiments shown, cast iron 20 has been poured aroundferrous sheath 28 in a cavity provided in lining 14 so that as itsolidiiies, it secures the ferrous sheaths 28 in proper position inlining 14. Other suitable means of binding the sheath to the lining maybe utilized however. For example, furane resins, or mixtures of furaneresins and carbonaceous materials can be utilized. A block 32 may beprovided at the opening of the U to help position copper core 25 withinferrous sheath 28.

In the embodiments shown in FIG- 3X ferrous sheath 28 is made of twoelements joined together in a suitable manner such as by welding to formthe desired downwardly opening U shaped cross section.

Magnetic energy cannot be blocked in the sense that electrical currentscan be by the interposing of an insulator in the conductor circuit. Itcan, however, be guided through a preferred path by shielding or theinterposing of an insulator or the interposing of a low reluctance pathbetween its source and the area from which it is desired to be excluded.This method, when correctly applied, can effectively reduce a givenlield and on the other hand, when unintentionally applied in the wrongpositions, magnetic materials can increase by nearly two times the fieldintensity at a given place.

As shown in the drawings, an adequate shield is interposed, i.e. ferroussheath 28 between the copper core 25 of the cathodic current collectors24 and the molten metal pad 16. At the same time, because of the greatlyreduced resistance of the copper core 25 as compared to a conventionalsolid ferrous current collector, the current flow is persuaded in themetal pad to a more vertical path thus improving the angle of incidence.

Advantageously, a portion of the ferrous sheath 28 near the center ofthe cell will be above the Curie point thus losing its magneticproperty. This increases the effectiveness of the instant invention. TheCurie point is the temperature of transition for a ferro-magneticsubstance at which the phenomena of ferro-magnetism disappears and thesubstance ybecomes merely paramagnetic.

In the embodiments shown, the copper core 25 may simply be slid intoposition within ferrous sheath 28. The elements would become squeezedtogether at operating temperatures due to the difference in coeicientsof expansion of the ferrous material and the copper. However, a returnto room temperature would permit the copper core 25 to be pulled out forreuse and repair as necessary.

Thus, an additional benefit from the instant invention is that the sideand end walls of the lining can be precast as part of the bottom blocks.This eliminates any ramming and requires only caulking of the seams. Thecopper cores can simply be removed from the sheaths and lining when thetime comes to reline the pot.

In summation, the copper core reduces the resistance such that thecurrent through a metal pad flows in a more vertical path.

The major portion of the current will tlow in the copper core thuspermitting the ferrous sheath of the cathodic current collector assemblyto function as an effective shield to reduce the magnetic field of theassembly to a much lower value at the point of interaction with themetal pad.

While there has been shown and described hereinabove possibleembodiments of this invention, it is to be understood that the inventionis not limited thereto and that various changes, alterations andmodifications can be made thereto without departing fromy the spirit andscope thereof as defined in the appended claims wherein- What is claimedis:

1. In an electrolytic cell for the production of aluminum whichcomprises:

(a) A shell;

(b) An anode suspended therein;

(c) A carbonaceous lining for the shell;

(d) A cathodic current collector embedded in the lining; the improvementwhich comprises utilizing for the cathodic current collectors a ferroussheath having a copper core so that the major portion of the currentWill flow out of the cell in the copper core and the ferrous sheathfunctions as a shield to reduce the magnetic field of the collector atthe point of interaction with the molten metal pad which forms on thelining during the operation of the cell, whereby the undesirablemovements and unequal elevation 5 of the molten metal pad formed on thelining during References Cited cell operation are reduced. 2. Theapparatus of claim 1 wherein the ferrous sheath UNITED STATES PATENTShas a substantially downwardly opening U shaped cross 215931751 4/1952Gfolee 204-243 section and the copper core is disposed within the bight2,846,388 8/1958 Mrel 204-243 of the U- 5 3,244,611 4/ 1966 Pezzack2-04-289 3. The apparatus of claim 1 wherein `the cathodic currentcollector is substantially horizontally disposed. HOWARD S' WILLIAMSPnmary Exammer `4. The apparatus of claim 1 wherein the ferrous sheathD. R, VALENTINE, Assistant Examiner is in the form of a hollow cylinderand the copper core 10 is in the form of a solid cylinder disposedwithin the ferrous sheath. 204-289 U.S. Cl. X.R.

